The present invention relates to a process for producing an energy cable. In particular, the present invention relates to a process for producing an energy cable for transporting or distributing electric energy, especially medium or high voltage electric energy, said cable having at least one thermoplastic electrically insulating layer.
Cables for transporting electric energy generally include at least one cable core. The cable core is usually formed by at least one conductor sequentially covered by an inner polymeric layer having semiconductive properties, an intermediate polymeric layer having electrically insulating properties, an outer polymeric layer having semiconductive properties. Cables for transporting medium or high voltage electric energy generally include at least one cable core surrounded by at least one screen layer, typically made of metal or of metal and polymeric material. The screen layer can be made in form of wires (braids), of a tape helically wound around the cable core or a sheet longitudinally surrounding the cable core. The polymeric layers surrounding the at least one conductor are commonly made from a polyolefin-based crosslinked polymer, in particular crosslinked polyethylene (XLPE), or elastomeric ethylene/propylene (EPR) or ethylene/propylene/diene (EPDM) copolymers, also crosslinked, as disclosed, e.g., in WO 98/52197. The crosslinking step, carried out after extruding the polymeric material onto the conductor, gives the material satisfactory mechanical and electrical properties even under high temperatures both during continuous use and with current overload.
To address requirements for materials which should not be harmful to the environment both during production and during use, and which should be recyclable at the end of the cable life, energy cables have been recently developed having a cable core made from thermoplastic materials, i.e. polymeric materials which are not crosslinked and thus can be recycled at the end of the cable life.
In this respect, energy cables comprising at least one coating layer, for example the insulation layer, based on a polypropylene matrix intimately admixed with a dielectric fluid are known and disclosed, for example, in WO 02/03398, WO 02/27731, WO 04/066317, WO 04/066318, WO 07/048422, WO 08/058572, and WO 11/092533. The polypropylene matrix useful for this kind of cables comprises polypropylene homopolymer or copolymer or both, characterized by a relatively low cristallinity such to provide the cable with a suitable flexibility, but not to impair mechanical properties and thermopressure resistance at the cable operative and overload temperatures. Performance of the cable coating, especially of the cable insulating layer, is also affected by the presence of the dielectric fluid intimately admixed with said polypropylene matrix. The dielectric fluid should not impair the above mentioned mechanical properties and thermopressure resistance and should be intimately and homogeneously admixed with the polymeric matrix.
For an industrial production of the above energy cables having a thermoplastic electrically insulating layer it is therefore necessary to envisage and develop a process which allows to homogenously admix the dielectric fluid with the thermoplastic material in a predetermined amount, without prejudicing stability of the extrusion process, which can be negatively influenced by the presence of the dielectric fluid in the early extrusion steps, when the polymer is not yet molten. In fact, because of the lubricant properties of the dielectric fluid, it can cause irregularities in the movement and plasticization of the polymeric material along the extruder barrel.
A possible solution to the above technical problem is described in the International Patent Application WO 02/47092, which relates to a process for producing a cable provided with at least one thermoplastic coating, which comprises: extruding a thermoplastic polymer and at least one dielectric liquid; passing said thermoplastic material through at least one static mixer, depositing and shaping said thermoplastic material around a conductor so as to obtain a layer of thermoplastic coating on said conductor. The addition of the dielectric liquid to the thermoplastic polymer is preferably carried out, as shown in the working examples, by injecting the liquid into the extruder in a zone wherein the polymer is already in a molten state, i.e. in a downstream zone of the extruder. Possibly, according to an alternative solution, the dielectric liquid may be added to the thermoplastic polymer when said polymer is in the solid state, namely: a) during the feeding of the thermoplastic polymer into the extruder, b) before the above feeding; or c) in a zone of the extruder wherein the thermoplastic polymer is in a solid state. In case b), the addition of the dielectric liquid can be carried out during a previous step of compounding the polymer in a mixer (batchwise or continuously) or by impregnating the polymer in the form of granules or powder. In any event, according to the disclosure of WO 02/47092, to obtain a dielectric liquid uniformly distributed throughout the cable coating, a homogenization step must be performed downstream of the extrusion step by means of a static mixer.
International Patent Application WO 02/27731 relates to a cable comprising at least one electrical conductor and at least one extruded covering layer based on a thermoplastic polymer material in admixture with a dielectric liquid. The mixing of the polymer base with the dielectric liquid may be carried out, for example, by an internal mixer having tangential or interpenetrating rotors, or by a continuous mixer, e.g. a Ko-Kneader (Buss) mixer or a co- or counter-rotating double-screw extruder. In the working examples, it is reported a method for obtaining cable specimens to be subjected to dielectric strength measurements, wherein the propylene homopolymer or copolymer in the form of granules and the dielectric liquid, along with an antioxidant, were fed into a double-screw extruder. The so obtained mixture was then passed into a single-screw extruder for further homogenization and filtered. The filtered mixture was then fed into another extruder, filtered again and then passed into a triple head for deposition, simultaneously with the semiconductive layers to form a triple layer on the metal conductor. Other dielectric strength measurements were performed on glass-shaped specimens of the above thermoplastic composition which were obtained by moulding discs of the insulating material which were previously produced starting from granules of the polymeric material impregnated with the dielectric liquid as follows. The polymer in granular form was preheated to 80° C. in a turbomixer, then the dielectric liquid was added to the granules under agitation at 80° C. for 15 minutes. After addition the agitation was continued for a further hour at 80° C. until the liquid was completely absorbed in the polymer granules. Afterwards, the material was kneaded in a laboratory double-screw at a temperature of 185° C. to complete homogenization. The material left the double-screw mixer in the form of granules, which were then compressed to form discs and then moulded to obtain the glass-shaped specimens.
U.S. Pat. No. 3,445,394 relates to a dielectric composition consisting of a solid phase polyolefin, in particular polyethylene, having dispersed therein an aromatic hydrocarbon oil and a voltage stabilizing additive. The additive-oil blends are also effective as voltage stabilizers in high density (low pressure) polyethylene and in other polyolefins, e.g. polyproylene. The blend of highly aromatic hydrocarbons and voltage stabilizing additive is used in the polyolefin in an amount effective to act as a voltage stabilizer, particularly an amount of from 1 to 10% by weight based on the amount of the polyolefin. The compositions are prepared by blending the oil and the stabilizer. Then the blend is added to a tumbling bin into which the polyolefin has previously been introduced. The polyolefin is granular and absorbs the blend upon tumbling. Subsequently the tumbled composition is shaped by extrusion to form wire insulation.
In GB Patent No. 1,303,334 an electric cable or wire is disclosed having an insulation comprising a solid olefin polymer together with one or more voltage stabilizers constituted by polymerisable aromatic or other cyclic monomeric compounds. These monomeric compounds may, for example, be introduced into the olefin polymer by impregnation of granular olefin polymer before extrusion. Even very small amounts of the voltage stabilizer, e.g. 0.1% of styrene, are sufficient to achieve a substantial improvement in the dielectric strength of the insulation.